1. Field of the Invention
This invention relates to an optical transmission fiber and to a process for producing the same.
2. Description of the Prior Art
Remarkable advances have been made in the development of low loss optical fiber for use in long-distance optical communications and commercial production of such fiber has already begun. The fiber uses a light-emitting diode (LED) or laser diode (LD) as a light source and is designed to permit low-loss, broad-band transmission. For this purpose, the fiber comprises a core having a diameter up to 50 .mu.m and a cladding having a refractive index that differs from that of the core by only about 1%.
An optical fiber has several advantages, such as lack of susceptibility to external induction, flexibility, light weight and large-capacity transmission. To take full advantage of these advantages, applications of optical fibers are expanding to computers, instrumentation, control systems and graphic communications. In these applications, there is a great demand for reducing the cost of manufacturing a transmission system comprising a light source, optical fibers and a light receiver or a system comprising such a transmission system plus branches and multiplexer circuits. Such systems use an inexpensive LED as a light source and achieve low system transmission loss by reducing the transmission loss in the coupling of the light source and the optical fiber. Needless to say, the fibers used in these systems must have high practical strength, and an equally important requirement is that the fibers have low transmission loss. Therefore, there is a demand for an optical fiber that does not have a transmission loss per kilometer of more than 10 dB, has a core diameter of about 100 .mu.m and is characterized by a differential refractive index between the core and cladding of about 2%. Conventionally, an optical fiber that is supposed to meet the demand has been produced in the following manner: a powder of a core glass (P.sub.2 O.sub.5 -GeO.sub.2 -SiO.sub.2) is laid over the starting member by flame hydrolysis and outside vapor-phase oxidation (OVPO) process, and the core glass is further overlaid with a powder of a cladding glass (B.sub.2 O.sub.3 -SiO.sub.2). After removing the starting member, the laminate is sintered and collapsed to provide a preform which is spun into a fiber. The fiber produced by such method is characterized by a core composed of a GeO.sub.2 -SiO.sub.2 or GeO.sub.2 -P.sub.2 O.sub.5 -SiO.sub.2 glass having a diameter of 100 .mu.m plus a cladding composed of a B.sub.2 O.sub.3 -SiO.sub.2 glass which, when having a diameter of 140 .mu.m (core diameter+clad diameter), provides a refractive index that differs from that of the core by about 2%. It is common practice to control the refractive indices of the core and cladding by mixing a dopant such as GeO.sub.2, B.sub.2 O.sub.3 or P.sub.2 O.sub.5 with the primary material SiO.sub.2. A high refractive index can be obtained using dopants such as P.sub.2 O.sub.5, GeO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2, but the only oxide available as a dopant for providing a low refractive index is B.sub.2 O.sub.3. A dopant has an effect on physical properties other than the refractive index of the primary material SiO.sub.2 ; it reduces the viscosity and increases the thermal expansion coefficient of pure SiO.sub.2 glass to which it is added. Therefore, if the core is doped with a large amount of GeO.sub.2 or P.sub.2 O.sub.5 in an attempt to increase the refractive index of the core, the viscosity of the core glass is reduced and the thermal expansion coefficient of the core glass is increased. Since the difference in residual stress between the core and cladding must be reduced to a minimum, the viscosity and thermal expansion coefficient of the cladding must be controlled so as to offset the change resulting from the decreased viscosity and increased thermal expansion coefficient of the core. Therefore, if the core is doped with a large amount of P.sub.2 O.sub.5 or GeO.sub.2 to increase the refractive index of the core, the cladding is likewise doped with a large quantity of B.sub.2 O.sub.3, but then the reduced viscosity and increased thermal expansion coefficient provide an optical fiber of low strength. In addition, the B.sub.2 O.sub.3 -SiO.sub.2 glass of which the outer cladding has low water resistance and great static fatigue in a moist or humid atmosphere is experienced. As mentioned above, B.sub.2 O.sub.3 is the only oxide dopant to provide a low refractive index. Thus, the cladding of all conventional optical fibers contains B.sub.2 O.sub.3 and hence has low water resistance.
One may say this problem can be solved by covering the cladding with a water-resistant glass jacket, but this idea is impractical because the state-of-art technology can seldom furnish a water-resistant glass composition whose viscosity and thermal expansion coefficient agree with those of a soft core and cladding containing a large quantity of dopant.